1. Field of the Invention
The invention relates to radiology systems that have a radiological film and are used to examine objects and, more particularly in such systems, it relates to a method that enables the estimation, while the object is being examined, of the "lumination" or "luminous exposure" (i.e. the quantity of light received multiplied by the exposure time) to which the radiological film is subjected, and enables the stopping of the exposure when the film has reached a given level of blackening or optical density.
2. Description of the Prior Art
A radiology system essentially comprises an X-ray tube and a receiver of such radiation, between which the object to be examined, for example a part of a patient's body, is interposed. The image receiver which is, for example, a film/screen couple, gives an image of the object after an appropriate exposure time and the development of the film. For the image of the object to be used as efficiently as possible, the different dots that constitute it should have sufficient contrast with respect to one another, namely, the blackening of the radiographic film should be appropriate from one X-ray image to the next one, despite the possible differences in opacity of the radiographed object.
The blackening of the film is related to the quantity of energy of the radiation incident to the film/screen couple, namely, the product of the intensity of the radiation to which the radiographic film is subjected, or "film" dose rate, by the time during which the film is exposed to this radiation. Consequently, to obtain a constant blackening of the film from one radiography to another, there is a known way of making measurements, during the examination, of the incident energy on the film by means of a detection cell, generally placed before the receiver, that is sensitive to X-radiation and gives a current proportional to the "film" dose rate. This current is integrated, from the start of the exposure, in an integrator circuit that gives an increasing value during the exposure. This increasing value is compared, during the exposure time, with a fixed reference value, established beforehand as a function of the characteristics of the film. The end of the exposure time is determined by the instant at which the comparison indicates that the value representing the incident energy on the film is equal to the reference value.
Should the radiographic film be directly subjected to X-radiation, and should the variation in the exposure times from one examination to another be small enough, a constant blackening of the film is obtained from one exposure to the next one, independently of the duration of the exposure time S, provided that the product of the exposure time S by the dose rate F is constant, i.e. the value resulting from the integration should remain constant.
This is true only if the characteristics of the film obey the law of reciprocity which indicates that the optical density of the film is proportional to the product F.times.S and if the response of the film is independent of the quality of the incident X-ray beam.
This law of reciprocity is no longer met when the variation in the exposure times is great.
Besides, should the radiographic film be associated with an intensifying screen, the blackening of the film depends on the quality of the spectrum. For, the response of the screen depends on the energy distribution of the spectrum of the radiation received, which means that it is sensitive to the hardening of the spectrum and to the change in voltage of the X-ray tube.
Finally, there are certain applications wherein it is costly for the detection cell to be placed before the film (for example in mammography) for the radiation energy is such that the detection cell would then be visible on the film. In this case, it is placed behind the image receiver but this creates an additional difficulty for the signal perceived by the detector cell is the one that has not contributed to the blackening of the film. The result thereof is that the measurement made by the detection cell does not generally represent the incident lumination on the radiographic film.
The deviation from the law of reciprocity, which varies according to the type of film, represents the relative variation of the lumination needed to obtain a constant optical density when the exposure time S varies while the spectrum of the X-radiation is constant. This is expressed by the fact that, to obtain a same optical density of the film, the lumination should be, for example 1 for an exposure time S=0.1 second, 1.3 for S =1 second and 2 for S=4 seconds.
This deviation from the law of reciprocity is due to the phenomenon known as the Schwarzschild effect. This effect is described notably in the work by Pierre GLAFKIDES, CHIMIE ET PHYSIQUE PHOTOGRAPHIQUES, 4th edition, pages 234 to 238, PUBLICATIONS PHOTO-CINEMA Paul MONTEL.
To account for this deviation from the law of reciprocity, various approaches have been proposed, and one of them has been described in the French patent No. 2 584 504. This patent proposes the comparison of the integrated value of the signal given by the detection cell with a reference value that varies during the exposure according to a determined relationship. More precisely, from the start of each exposure period, an additional value is added to the difference between the values of the integrated signal and of the reference value. This additional value increases as a function of time according to a previously determined relationship, for example an exponential relationship.
This previously determined relationship, whether it is exponential or otherwise, takes account of the deviation from the law of reciprocity only imperfectly. In particular, it does not take account of the variations in the luminous intensity effectively received by the film.
Furthermore, this correction does not take account of the effects of other phenomena such as the hardening of the X-radiation due to the thickness of the object crossed and the modification of the spectrum due to the voltage of the X-ray tube.
Furthermore, in this method, the detection cell is placed before the image receiver.
An object of the present invention, therefore, is to implement a method for the automatic determination, during the time of exposure, of the instant when the exposure is stopped, taking account of the different effects that come into play, notably the variations in the tube current, the hardening of the spectrum due to the thickness of the object crossed, the modification of the spectrum due to the voltage of the tube and, when an intensifier screen is present, the absorption response of said screen.